Alloys for Cast Metal Restorations

Question 1

What are the two alloys used for cast metal restorations?

Answer 1

• Crown and bridge alloys- Porcelain fused-to-metal alloys

Question 2

What is a PFM crown?

Answer 2

• Porcelain surface- Metal alloy substructure - Porcelain fused metal crown

Question 3

Why must porcelain be bonded to a metal alloy?

Answer 3

• Porcelain has good aesthetics - But microcracks tend to form at fitting surface due to large biting forces - Makes it prone to mechanical failure- Alloys withstand large stresses readily as have good mechanical properties

Question 4

What is compressive strength?

Answer 4

• Stress to cause fracture

Question 5

What is elastic modulus (rigidity)?

Answer 5

• Stress/strain ratio- Stress required to cause change in shape

Question 6

What is brittleness/ductility?

Answer 6

• Dimensional change experienced before fracture

Question 7

What is hardness?

Answer 7

• Resistance of surface to indentation or abrasion

Question 8

What can be ascertained from a stress -strain curve and what can’t?

Answer 8

• Strength (compressive/tensile)- Brittleness/ductility- Elastic modulus (rigidity)- Hardness is not

Question 9

What are some key features of a stress strain curve?

Answer 9

• Initial gradient = Elastic modulus (steeper gradient means more rigid)- Red dots at end of curve = fracture stress- If small gap between fracture stress and proportional limit = brittle- If large gap between FS and PL = ductile

Question 10

What materials tend to be brittle and what tends to be ductile?

Answer 10

Brittle = CeramicsDuctile = Alloys

Question 11

What are the properties of procelain?

Answer 11

• Hard so surface withstands abrasion/indentation well- Quite rigid so large stress required to cause strain- Strong so high compressive strength- Low tensile strength so tendency to form surface defects leads to fracture at low stress- Quite brittle so low fracture toughness- Not ductile- Maximum strain is approx 0.1% before fracturing

Question 12

What are the properties of porcelain-fused alloys?

Answer 12

• Alloys much stronger- Much harder and more rigid- More ductile- Withstand greater degrees of permanent strain when subjected to large stresses like biting

Question 13

What is the structure of porcelain-metal restorations?

Answer 13

• Metal oxide bonded to both the porcelain and the alloy

Question 14

What is the purpose of the metal oxide?

Answer 14

• Helps eliminate defects/cracks on porcelain surface

Question 15

What is the purpose of the alloy?

Answer 15

• Alloy supports and limits strain that porcelain experiences- More rigid so change shape very little and return to original dimensions- Helps it not reach level for brittle failure

Question 16

To avoid developing defects or micro-cracks what must the porcelain-fused-ally undergo?

Answer 16

• Both porcelain and alloy should have similar thermal expansion coefficients - Due to needing to be fired in furnace then cooling - If didn’t have similar then defects would occur- So they expand at same rate when heated and contract at same rate when cooled

Question 17

What are some different alloys that have been developed for bonding to porcelain?

Answer 17

• High gold alloy- Low gold alloy- Silver palladium (AgPd)- Nickel chromium (NiCr)- Cobalt chromium

Question 18

Why must the alloy form a good bond to porcelain?

Answer 18

• AKA good wetting- Means goo surface contact- Porcelain forms bond with metallic oxides on surface- To ensure the restoration does not fall apart in patients mouth and swallow

Question 19

Why must the thermal expansion coefficient of alloy be similar to porcelain?

Answer 19

• Porcelain is 14ppm per degree C- To avoid setting up stresses during fusing of porcelain to alloy- So defects and micro-cracks don’t form on cooling stage

Question 20

What must a property of the alloy be to ascertain good aesthetics?

Answer 20

• Needs to avoid discolouration of porcelain as porcelain has good aesthetics- Ag in AgPd can produce green discolouration- Copper not used in high gold alloy for this reason

Question 21

What mechanical properties are desirable of an alloy?

Answer 21

• High bond strength- Good hardness- High elastic modulus (rigid) to support porcelain and prevent fracture

Question 22

What must the melting, recrystallisation temp of alloy be compared to porcelain?

Answer 22

• Must be higher than fusion temp of porcelain- Or creep may occur

Question 23

What is creep?

Answer 23

• Gradual increase in strain (permanent) experienced under prolonged application of stress (

Question 24

What are the constituents of High gold alloys?

Answer 24

• Gold (Au) = 80%- Platinum/Palladium (Pt/Pd) = 14% - Silver (Ag) = 1%- Small amount Indium, Tin- No Copper (green hue)

Question 25

Why is Pt/Pd used in high gold alloys?

Answer 25

• Matches thermal expansion of porcelain- Increases its melting point which Helps minimise potential for creep

Question 26

Why is Indium and Tin used in high gold alloys?

Answer 26

• Enable metal oxide layer to form- Enables bonding to porcelain

Question 27

What are the disadvantages of high gold alloys?

Answer 27

• Melting range too low- Young’s modulus too low (not rigid)

Question 28

What are the constituents for low gold alloy?

Answer 28

Au = 50%Pd = 30%Ag = 10%Indium, Tin = 10%

Question 29

Why is low gold better than high gold?

Answer 29

• Increased melting temperature- Slightly better mechanical properties

Question 30

What are the constituents of silver palladium alloys?

Answer 30

Pd = 60%Ag = 30%Indium, Tin = 10%

Question 31

A negative and positive about silver palladium alloys?

Answer 31

• High melting point- Care needed in casting and is challenge for technicians

Question 32

What are the constituents of Nickel-chromium alloys?

Answer 32

Ni = 70-80%Cr = 10-25% (creates oxide bond)

Question 33

What are the advantages and disadvantages for Nickel-chromium alloy?

Answer 33

• High melting point- High young modulus (rigid)But - has high casting shrinkage so challenging to use- Low-ish bond strength to porcelain

Question 34

What are the advantages and disadvantages to Cobalt-chromium alloys?

Answer 34

• High melting point (1300C-1400C)- High young modulus (220 GPa)- High tensile strength (850MPa)- High hardness (360-430 VHN)But- Low ish bond strength (220GPa)- Casting shrinkage 2.3%

Question 35

Why are there biocompatibility concerns for nickel chromium?

Answer 35

• Allergy to nickel

Question 36

What alloys have high casting shrinkage?

Answer 36

• AgPd- NiCr- Cocr

Question 37

What is the most used alloy in GDH labs?

Answer 37

• CoCr

Question 38

What are the 3 mechanisms for bond between oxide layer, alloy and procelain?

Answer 38

• Mechanical- Stressed skin- Chemical

Question 39

What is the mechanical bond between oxide layer, alloy and porcelain?

Answer 39

• Due to surface irregularities on alloys metal oxide layer and porcelain- Allows them to interlock- Least important of the 3

Question 40

What is the stressed skin effect?

Answer 40

• Depends on slight differences in thermal contraction coefficients- After furnace stage, alloy contracts slightly on cooling- Generates compressive forces on porcelain- Aids bonding

Question 41

What is the chemical bond?

Answer 41

• Electron sharing in metal oxide coating alloy and the porcelain- Occurs during firing of porcelain where high temps reached- Described as electron sharing

Question 42

What are the modes of failure in porcelain fusion?

Answer 42

• Oxide layer fracturing- Oxide layer delaminating from alloy- Porcelain detaching from oxide layer (most ideal situation)